1. Field of the Invention
The present invention relates to a cellulose acylate laminate film and a method for producing a cellulose acylate laminate film, and to a polarizer and a liquid crystal display device.
2. Description of the Related Art
In recent years, TV application of liquid crystal display devices is being more popular, and with TV screens getting larger, quality enhancement and cost reduction of display devices are much required. In particular, VA-mode liquid crystal display devices have high contrast and their production yield is relatively high, and therefore, they are the most popular for liquid crystal display devices for TV use.
However, VA-mode liquid crystal display devices have a problem in that, at the time of black level of display, the VA-mode liquid crystal display devices could exhibit almost complete black images in the normal direction of the display panel, but when the black-level panel is observed in oblique directions thereof, then there occurs light leakage that makes background black expression impossible, and therefore the viewing angle is narrowed. Accordingly, in recent years, a liquid crystal display device having a high contrast and having further improved viewing angle characteristics is desired.
For solving the problem of viewing angle characteristics, for example, there is proposed a method of reducing the light leakage in observation of a liquid crystal display device in oblique directions at the time of black level of display, by incorporating therein a combination of a first retardation film having a positive monoaxial refractive anisotropy and a second retardation film having a negative refractive anisotropy of which the thickness-direction refractive index is fully smaller than the in-plane refractive index thereof (for example, Japanese Patent 3027805).
However, the method described in Japanese Patent 3027805 could reduce the light leakage only within a specific wavelength range (for example, green light at around 550 nm), in which, therefore, there still occurs light leakage in the other wavelength region where the wavelength dispersion characteristics of the retardation film and the wavelength dispersion characteristics of the liquid crystal cell are incompatible with each other (for example, blue light at around 450 nm, and red light at around 650 nm). Accordingly, when the liquid crystal display device described in Japanese Patent 3027805 is observed in oblique directions at the time of black level of display, then the panel is colored in blue and red, therefore bringing about a problem of color shift. Therefore, the method described in Japanese Patent 3027805 is not still satisfactory from the viewpoint of solving the problem of viewing angle characteristics.
As a method of solving the problem of color shift in observation of liquid crystal display devices in oblique directions at the time of black level of display, it is investigated to improve the wavelength dispersion characteristics of the film for use in the retardation film in order that the wavelength dispersion characteristics of the retardation film could be compatible with the wavelength dispersion characteristics of the liquid crystal cell. As a method of improving the wavelength dispersion characteristics of the retardation plate, disclosed is a method of stretching a film of a cellulose acetate having a specific degree of acetylation to give a cellulose acetate film of which the birefringence is larger at a longer wavelength (see Japanese Patent 3459779).
Use of the film of which the birefringence is larger at a longer wavelength described in Japanese Patent 3459779 could solve the problem of color shift in some degree; however, the film could control only the wavelength dispersion characteristics of retardation but rather brings about another problem of haze increase. When a film having a problem of high haze is incorporated in a liquid crystal display device, then the contrast ratio of the image to be displayed lowers, and therefore, the problem to satisfy the two requirements of high contrast and improved viewing angle characteristics is not still solved.
On the other hand, JP-A 2003-33998 and JP-A 8-207210 disclose a cellulose ester laminate film having a specific laminate structure and having a reduced haze. In these references, in addition to the haze of the film, the other characteristics such as the moisture permeability and the dimensional stability thereof are investigated; however, the wavelength dispersion characteristics of retardation of the film are not investigated at all.